1. Field of the Invention
The invention relates to a method for ultrasonic imaging of an organ in a patient's body through a part of the patient's respiratory tract.
2. Description of Related Art
Such a method is known from WO 00/53098. This prior art document relates to an ultrasonic imaging method that is known as transesophageal echocardiography (TEE). TEE has become a widely used imaging technique for evaluating cardiac structure, function, and valvular anatomy. TEE has also provided a new perspective on the thoracic aorta, and there is growing evidence that the technique contributes valuable and sometimes unique information about aortic structure and pathology.
TEE involves introducing an echo probe into the patient's esophagus and transmitting ultrasound waves across the thorax in the direction of the heart and aorta. However, visualization of the ascending aorta by internal TEE is limited by an air structure, i.e. the trachea and main left and right bronchi. This is due to an important physical limitation of ultrasound: absorption of ultrasound waves. This absorption is dependent of the medium and expressed in terms of the “half power distance”: the distance in which half of the ultrasound energy will be absorbed. For water this is 360 cm, bone 0.2 cm and for air 0.06 cm. This means that in practice ultrasound waves will not travel through bone or air.
Unfortunately, by the anatomical location of the aorta ascendens and the upper part of the main vascular side branches, it is difficult to view this area by TEE because the view is obstructed by the trachea. The trachea is located between the esophagus and the vascular tree, so all echoes are reflected by the trachea, which is filled with air.
In order to solve this problem, WO 00/53098 proposes the use of a balloon that may be arranged in the trachea or in one of the bronchi and that may be filled with an ultrasonic transmission fluid, e.g. water or a saline solution in minor concentrations. Obviously, this can only be done during operative surgery, when the patient is mechanically ventilated or on cardiopulmonary bypass, since in order to be effective the balloon has to completely fill and block the trachea or bronchus.
A problem which arises when trying to introduce the balloon in the left bronchus, which is the position of choice when visualizing the aorta ascendens, is that the flexible catheter carrying the balloon is hard to manipulate. Therefore, positioning the distal end of this flexible catheter in front of the left bronchus, so that the balloon may be lowered into that bronchus, is often a matter of trial and error. Since this positioning has to be performed during operative surgery, when timing is often critical, there is a clear need for an improved imaging method.